Along with the increase of aquaculture of fish, there have been many investigations into the vitamin requirements of aquacultured fish. As a result, it has been found that vertebral lordosis, vertebral scoliosis, and fracture and dislocation of vertebra occur in rainbow trout, channel catfish, coho salmon, young yellowtail, striped beakperch, etc. fed with L-ascorbic acid (vitamin C) deficient feed, and that bleeding in fins and jaws, and damage of gill cover occur in Japanese eel fed with said feed.
The symptom common to those L-ascorbic acid deficient fish is the failure of connective tissue. In order to prevent the symptom, vitamins including L-ascorbic acid are added in the feed for fish rearing, and fed to fish (for example, Japanese Patent Publication No. 2195/1973 and Japanese Patent Publication No. 12573/1988).
Nevertheless, L-ascorbic acid is particularly unstable among water soluble vitamins, and the decrease of the activity of L-ascorbic acid by decomposition when added with fish meal and minerals in feed is left to be solved.
The required amount of L-ascorbic acid has been investigated, and found to be 10-20 mg/kg feed at minimum.
In the meantime, the aquaculture of crustaceans such as kuruma prawn, giant tiger shrimp, Oriental river prawn and swimming crab has been increasing in recent years. The ascorbic acid requirement of crustaceans has also been investigated, and found to be, for example, 300-1000 mg/100 g feed on the L-ascorbic acid basis for young kuruma prawn.
The crustaceans are incapable of synthesizing L-ascorbic acid, and need to take it from feed. To meet this demand, feed for the aquaculture of crustacean, which contains L-ascorbic acid is being marketed.
This feed, however, is heat-processed at 100.degree. C. or more during pelleting, and L-ascorbic acid added in the feed is decomposed to a considerable extent.
In addition, prawns hold and gnaw feed by nature. Since prawns take extended time to eat in this way, the dissolution of L-ascorbic acid from feed becomes great, and it is said in one report that after all, only 1/10 of the administered amount can be taken in after destruction during the production and storage of feed.
Another report says that the survival rate of crustaceans such as kuruma prawn shows a drastic fall when they are fed with L-ascorbic acid deficient feed.
Although the feed on the market is produced with an excessive amount of L-ascorbic acid charged in before pelleting, L-ascorbic acid is easily decomposed not only by heat but also by metals such as copper in fish meal and feed, and more than 70% of the amount added is decomposed during the production of the feed. Moreover, L-ascorbic acid has poor stability during storage of feed.